Black hole awaits its dinner date with a gas cloud

Mar. 26, 2014
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These images from NASA's Chandra X-ray Observatory home in on the supermassive black hole at the center of the Milky Way galaxy. The large image combines X-ray images from Chandra with infrared images from Hubble Space Telescope. Inset just shows X-ray emissions, covering a region that's half of a light year wide. / X-ray: NASA/UMass/D. Wang et al; IR: NASA/STSci

by Traci Watson, Special for USA TODAY

by Traci Watson, Special for USA TODAY

The black hole at the center of our galaxy must be grumpy. For years, it has had almost nothing to eat except perhaps a few comets and the like, a starvation diet that has turned its light display into a faint shadow of what it should be.

Now it might want to tie on its napkin. A dusty cloud of gas - a potential feast - is plummeting through space straight toward the black hole's waiting maw. The middle of the cloud is scheduled to make its closest approach to the black hole anytime now. If the black hole is lucky, it will gulp down the gas, possibly lighting off a spectacular fireworks display of radiation and giving scientists an unprecedented view of a black hole's table manners. But the gas cloud may not be what it seems - which would condemn the ravenous black hole to continue its fast.

"I would not be surprised if we see something," says the first person to spot the gas cloud, Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics. But then he clarifies, "I would not bet my house on that."

"Will there be fireworks or not? The clear answer has to be 'Maybe,' " astrophysicist Leo Meyer of the University of California-Los Angeles said at a scientific meeting this year. "We just don't know. â?¦ But it's still very much worth looking."

If there's a fireworks display, backyard sky watchers are out of luck: Only the world's most advanced telescopes will detect the display. But high-tech observatories should be able to release pictures of any black-hole meal in nearly real time. NASA's Swift satellite, for example, points itself at the galactic center every day, producing images that are quickly posted online for all to see.

What no one else will be able to see is the black hole itself. Nothing can escape a black hole, not even light, so direct observation is impossible. But the motion of the stars near our galaxy's black hole provides some details about it, as does the radiation emitted by hapless objects spiraling into the hole.

These clues suggest the black hole at the center of our Milky Way galaxy is a whopper of an object, roughly 4 million times the mass of our sun, earning it a place in the "supermassive" category of black holes. Yet the radiation wafting from the region around the black hole is oddly dim. Given the black hole's mass, its collar of radiation is eight to nine times fainter than if the black hole were eating at full strength, says Nathalie Degenaar of the University of Michigan. Black holes, like babies, grow by eating. So scientists are baffled by the mismatch between the black hole's titanic size and its restricted rations.

In 2011, scientists spotted a strange and wonderful object at the galaxy's center. It was a small, blob-like cloud of hydrogen gas, and it was in the early stages of being ripped apart by the black hole's tremendous gravitational pull. Eventually, bits of cloud could spill into the black hole, the cloud's discoverers said - something researchers have never witnessed at close range. Excited scientists began enlisting telescopes around the world to follow the gas cloud on its journey to doom.

As the gas cloud drew closer and closer to its fate, the black hole's tug "spaghettified" the cloud into a thin, noodle-shaped body about 150 billion miles long. Simulations show that the stretched-out cloud is beginning to circle the black hole, and the gas has probably started to mix with the black hole's thin atmosphere, Gillessen says. If gas starts to rain into the black hole, showers of bright X-rays or other radiation could light up the center of the galaxy, perhaps in the next few months, perhaps within a few years.

Or maybe not at all.

Scientists have come to realize that what looks like a plain cloud of gas may be a star disguised by a gassy cloak, a possibility that could be a serious spoiler. A gas cloud-star combination would put up more of a fight against the black hole's tug than a cloud of pure gas, which could mean less material dribbling into the black hole. A big meal for the black hole might trigger the flashy radiation spikes that researchers dearly want to see, but a small snack won't.

If the cloud includes a hidden star, scientists will see "probably nothing," Degenaar says. "It will just pass, it will just go on with its life â?¦ and leave us disillusioned."

The object concealed inside the cloud could be a star girdled with a disk of planet-forming material. Maybe it really is a pure gas cloud. Maybe, says astrophysicist Andrea Ghez of UCLA, it's the product of two stars that have recently merged, but she concedes that there are more proposals than evidence.

"There are all sorts of ideas out there, and none of them works perfectly," she says. "I have my own favorites, but they're just as unlikely as any other scenario."

Whatever the true nature of the gassy cloud, some of the world's most advanced observatories are on the lookout for the first sign of activity. Telescopes in Hawaii, satellites in space and radio dishes in Japan are being trained on the center of the galaxy in what Ghez calls "a race to see what's happening. Everyone's going to be trying to be first."

She and other researchers are still holding out hope that the cloud will serve up a substantial meal to the black hole, which would allow astronomers to see in detail for the first time how a black hole eats. No other black hole in the universe is close enough to provide such a view. But whether there are fireworks or not, close study of the gas cloud as it swerves close to our black hole should pay off.

"Super-massive black holes are fascinating objects, and we know so profoundly little about them," Ghez says. "There's huge ignorance here, which is an opportunity."